The present invention is generally related to control of electrodynamo machines, and, more particularly, to system and method for controlling a synchronous machine using a changeable cycle-conduction angle.
Propulsion drive systems for self-propelled pieces of equipment, including electric vehicles or hybrid electric vehicles, need to have a relatively wide speed range, high torque per ampere, high efficiency, quick dynamic response, and operational robustness and reliability under tough environmental or operational conditions.
Synchronous machines are commonly used in a large variety of applications, including automotive applications. For example, the automotive generator is commonly a xe2x80x9cLundellxe2x80x9d type wound-field synchronous machine. Its traditional application has been as a generator for charging the vehicle battery and providing 12V accessory power in the vehicle. This is accomplished by connecting the generator""s stator windings to a three phase diode rectifier. The rectified DC output is fed to the battery. An electronic regulator controls the excitation current to control the rectified DC output voltage from the diode bridge.
It is known that such a machine can also be used as an electric starter motor (or cranking motor) of the internal combustion engine in the vehicle, assuming the diode bridge is replaced with an active semiconductor bridge, typically comprised of metal oxide semiconductor field effect transistors (MOSFETs). In this configuration, the electric machine can perform the role of both a cranking motor as well as a vehicle electric generator. FIG. 1 described below is illustrative of one known implementation.
FIG. 1 shows a three-phase electrodynamo machine 1. FIG. 1 further shows a circuit, such as a control and rectifier bridge circuit 2, and a unit 3 for controlling the bridge circuit 2. In conventional manner, the machine 1 includes: a coil-carrying rotor 4 constituting the primary magnetic circuit associated with the rings and brushes that convey excitation current (of the order of a few amps); and a polyphase stator 5 carrying a plurality of coils constituting the secondary magnetic circuit, connected in star or delta configuration in the common case of a three-phase structure and acting, during generator operation, to deliver converted electrical power to the rectifier bridge 2 (several tens of amps at a voltage of the same order as the battery voltage).
The bridge circuit 2 is connected to the various phases of the secondary magnetic circuit 5 and is connected between ground and a power supply terminal of the battery B of the vehicle. It is constituted by a plurality of diodes 6 forming a rectifier bridge, and also by a plurality of upper and lower switching devices 7, such as MOSFETs, that are connected in parallel with respective diodes 6 and control the various phases of the machine. In motor mode, the diodes act as freewheel diodes, whereas in generator mode, they act as a rectifier bridge. The MOSFET can also be energized during generation to conduct backwards. This mode of operation is commonly referred to as synchronous rectification and generally increases the converter efficiency with the FET body and diode conducting in parallel. The motor mode operation of such a machine is achieved by applying DC field excitation current to the primary magnetic circuit 4 and by delivering signals that are phase-shifted by 120 degrees to the phases of the stator.
In order to produce continuous torque for motoring, a position sensor is provided to synchronize the energization of the phase windings with the rotation of the machine. In its simplest form, for a three-phase machine this sensor is comprised of three position sensors that are spatially located 60 or 120 electrical degrees from another. This arrangement is commonly utilized in the control of brushless DC permanent magnet machines (BDCPM). One mode of machine excitation that has been typically used in BDCPM machines is 120 degree conduction. That is, a cycle-conduction mode wherein each of the six upper and lower inverter switching devices conducts for 120 electrical degrees per cycle. This conduction mode is known to result in the highest machine torque per ampere (amp) ratio under most circumstances. Another mode of machine excitation typically used is 180 degree conduction, or six-step excitation. That is, a. cycle-conduction mode where each of the six inverter devices for a three-phase machine conducts for 180 electrical degrees per cycle. Machines driven with 180 degree excitation generally have a lower torque per amp ratio than those with 120 degree excitation, and hence have a higher no-load speed. Unfortunately, it is believed that present control techniques for electrodynamo machines used in automotive equipment both as a generator, and as a motor for starting an engine mechanically coupled to the machine, have failed to recognize that one could appropriately combine the advantageous torque characteristics provided by the 120 degree excitation with the higher no-load speeds provided by the 180 degree excitation in order to quickly and reliably facilitate occurrence of a succesful cranking event, even under demanding environmental and operational conditions.
In view of the foregoing isssues, it is desirable to provide synchronous machine control techniques that start the machine in a first mode of cycle-conduction that enables the machine to overcome the frictional and compressive forces of the internal combustion engine coupled to the starter system. Upon the machine reaching a desired speed, it would be further desirable to switch to a second mode of cycle-conduction that improves the no-load speed performance of the machine. It would be further desirable to be able to quickly and reliably start and control the machine without resorting to complex control algorithms, expensive sensors or without having to perform burdensome hardware modifications to the machine.
Generally, the present invention fulfills the foregoing needs by providing in one aspect thereof a method for controlling a synchronous machine including a polyphase stator. The machine may be used in a piece of equipment respectively as a generator and as a motor for starting an engine in the piece of equipment. The method allows to sense rotor position of the machine using a sensor assembly configured to supply a respective stream of pulses indicative of rotor position relative to each phase of the machine. The method further allows to process each stream of pulses from the sensor assembly to generate a first set of inverter control signals. The first set of inverter control signals is applied to an inverter circuit coupled to energize the phases of the stator to start the machine. The first set of control signals is configured to provide a first cycle-conduction angle relative to the zero-crossings of the respective phase EMF voltages of the machine to produce a sufficiently high level of torque during start of the machine. Upon the rotor reaching a predefined rotor speed value, each stream of pulses from the sensor assembly is processed to generate a second set of inverter control signals. The second set of inverter control signals is applied to the inverter circuit to energize the phases of the stator. The second set of control signals is configured to provide a second cycle-conduction angle relative to the zero-crossings of the respective phase EMF voltages of the machine to enable a sufficiently high rotor speed and thus facilitate the occurrence of a successful cranking event for the engine of the piece of equipment.
The present invention further fulfills the foregoing needs by providing in another aspect thereof a system for controlling a synchronous machine including a polyphase stator. The system includes a sensor assembly configured to supply a respective stream of pulses indicative of rotor position relative to each phase of the machine. The system further includes a processor configured to process each stream of pulses from the sensor assembly to generate a first set of inverter control signals. An inverter circuit is coupled to receive the first set of inverter control signals and energize the respective phases of the stator to start the machine. The first set of control signals is configured to provide a first cycle-conduction angle relative to the zero-crossings of the respective phase EMF voltages of the machine to produce a sufficiently high level of torque during start of the machine. Upon the rotor reaching a predefined rotor speed value, the processor is configured to process each stream of pulses from the sensor assembly to generate a second set of inverter control signals. The second set of control signals is configured to provide a second cycle-conduction angle relative to the zero-crossings of the respective phase EMF voltages of the machine to enable a sufficiently high rotor speed and thus facilitate the occurrence of a successful cranking event.